In a cellular system, a mobile station normally performs a cell search processing to seek a cell which connects a radio link.
The cell search is executed using a synchronization channel (SCH) included in a radio frame in a downstream link. In addition to the synchronization channel, a cell-specific pilot channel and a broadcast channel (BCH) may also be used (Non-patent Document 1: 3GPP TR 25. 814 V7.0.0). An example of the cell search will be described with reference to the drawings.
FIG. 1 indicates an example of a configuration of a radio frame transmitted from a base station transmission apparatus.
As FIG. 1 indicates, the radio frame is constructed by various channels multiplexed in a two-dimensional direction of time and frequency. In the example in FIG. 1, the radio frame has 10 sub-frames, SF1 to SF10, in the time direction, and each sub-frame SF consists of two slots: the first half slot and the latter half slot.
In each slot, a resource uniquely determined by a symbol position (time) and a sub-carrier position (frequency) is called a “resource element”.
The various channels multiplexed in a slot includes a primary synchronization channel (P-SCH), a secondary synchronization channel (S-SCH) and a pilot signal channel (P-CH).
The primary synchronization channel (P-SCH) has a common pattern for all the cells, and is time-multiplexed in the end symbols of the first half slot #0 of the first sub-frame SF1 and of the first half slot #10 of the sixth sub-frame SF6 respectively.
The secondary synchronization channel (S-SCH) has a pattern, which is specific to a cell ID group, and which is a group of cell IDs assigned to each cell in advance. The secondary synchronization channel (S-SCH) is time-multiplexed in the second symbol from the respective ends of the first half slot #0 of the first sub-frame SF1 and of the first half slot #10 of the sixth sub-frame SF6.
The pilot signal channel (P-CH) also has a cell-specific scramble code which is information specific to a cell, and is time-multiplexed in the first symbol and the fifth symbol of each slot (#0, #1, #2, . . . ).
The cell ID assigned to each cell and the cell-specific scramble code correspond one-to-one, so the mobile station can determine a cell ID of a cell in which the mobile station is located by specifying the cell-specific scramble code.
For the cell-specific scramble code, a method of using a sequence of a base station-specific pseudo-random number sequence multiplied by a phase rotation sequence, which is orthogonal between sectors within a same base station, or a method of using a generalized chirp like sequence for the pseudo-random number sequence, for example, may be used.
FIG. 2 illustrates a cell search processing procedure performed in a mobile station. When the radio format depicted in FIG. 1 is received from a base station, the mobile station detects the correlation with a replica of a time signal of the primary synchronization channel (P-SCH), which is a known pattern, as a processing in the first step, and decides a timing indicating the maximum correlation value, for example, as the sub-frame timing (step S1).
As the second step, fast Fourier transform (FFT) processing is performed at the timing detected in the first step, so that the received radio format is transformed into a frequency domain signal, and the secondary synchronization channel (S-SCH) is extracted from the frequency domain signal. Then correlation of the extracted secondary synchronization channel (S-SCH) and each candidate secondary synchronization channel sequence replica is determined, and a candidate secondary synchronization channel sequence having a maximum correlation value, for example, is decided as a detected secondary synchronization channel sequence. A cell ID group is determined by the detected secondary synchronization channel (step S2).
As the third step, fast Fourier transform (FFT) processing is performed at the timing detected in the first step so that the signal is transformed into a frequency domain signal, and the pilot signal channel (P-CH) is extracted from the transferred frequency domain. Then the extracted pilot signal channel (P-CH) is correlated with a scramble code replica corresponding to each candidate cell ID included in the cell ID group detected in the second step, and a cell ID corresponding to a candidate scramble code indicating a maximum correlation value, for example, is decided as a detected cell ID (step S3). By this, a cell in which the mobile station is located may be specified.
In the case of 3GPP (Third Generation Partnership Project), specifications of the multimedia broadcast/multicast service (MBMS) are under consideration, aiming at standardizing the next generation portable telephone communication.
For example, MBMS data is time-multiplexed with the unicast data in sub-frame units. The Non-patent Document 1 describes a method for improving the reception quality by using a guard interval, which is longer than the guard interval used for unicast data, transmitting a same data from a plurality of cells at a same timing using a same frequency, and combining received signals at a mobile station side.
This is called a “single frequency network”. In this case, a same cell-common pilot signal among cells is transmitted for demodulating the same MBMS data transmitted from a plurality of cells.
The Non-patent Document 2 describes that the control signal for a unicast is multiplexed with a sub-frame allocated to MBMS data (hereafter called MBMS sub-frame), and a cell-specific pilot signal having a different pattern in each cell for unicast is multiplexed with the MBMS sub-frame for demodulating the control signal for unicast and measuring CQI.
A configuration of a pilot signal of an MBMS sub-frame is also described in Non-patent Document 3. According to this configuration, a cell-specific pilot signal for unicast is multiplexed only with a first symbol of an MBMS sub-frame.
In the case of time-multiplexing an MBMS sub-frame, as mentioned above, sub-frames having different guard interval lengths are time-multiplexed. In an initial cell search which is executed when power of the mobile station is turned ON, a problem occurs in the above mentioned third step of a cell search, since information on the guard interval length of the receive sub-frame is not available.
This problem is described in detail in Non-patent Document 4. One means for solving this problem is to improve a method for attaching a guard interval of MBMS sub-frames, as described in the Non-patent Document 4. Another method is using, as indicated in non-patent Document 5, only pilot signals in a sub-frame in which a synchronization channel has been multiplexed in the initial cell search.    Non-patent Document 1: 3GPP TR 25. 814 V 7.0.0    Non-patent Document 2: 3GPP TSG-RAN WG1, R1-060372, “Multiplexing of Unicast Pilot and Control Channels in E-MBMS for E-UTRA Downlink”, Texas Instruments    Non-patent Document 3: 3GPP TSG-RAN WG1, R1-070383, “Reference Signals for Mixed Carrier MBMS”, Nokia    Non-patent Document 4: 3GPP TSG-RAN WG1, R1-060563, “Channel Design and Long CP Sub-frame Structure for Initial Cell Search”, Fujitsu    Non-patent Document 5: 3GPP TSG RAN WG1, R1-063304, “Three-step Cell Search Method for E-UTRA”, NTT DoCoMo, Institute for Infocomm Research, Mitsubishi Electric, Panasonic, Toshiba Corporation.
If MBMS sub-frames are multiplexed in a radio frame, a number of resource elements of cell-specific pilot signals in one radio frame decreases, compared with a case of assigning only unicast sub-frames to the radio frame (this relationship may be reversed in some cases).
The number of resource elements of cell-specific pilot signals in one radio frame also depends on the number of MBMS sub-frames that are multiplexed. For example, if a cycle of scramble codes of cell-specific pilot signals is one radio frame, then the phase of the scramble code at each transmission timing of the cell-specific pilot signal changes by multiplexing the MBMS sub-frames.
FIG. 3 illustrates a case of allocating all the sub-frames of a radio frame to unicast (case 1), and a case of allocating the sub-frames #1 and #4 to MBMS (case 2) as examples.
In FIG. 3, the column “phase of cell-specific scramble code” is based on the assumption that the cell-specific scramble code is a cell-specific pilot signal, and resource elements allocated to the cell-specific pilot signal are listed from one at the lower frequency side, and are indicated by a phase of the cell-specific scramble code allocated to the resource element at the lowest frequency side at each transmission timing of the cell-specific pilot signal.
Np denotes a number of resource elements allocated to the cell-specific pilot signal in each symbol of the cell-specific pilot signal.
In case 1, where all the sub-frames are allocated to unicast, the phase shift of the cell-specific scramble code does not occur.
In case 2, on the other hand, the sub-frames #1 and #4 are allocated to MBMS, so a phase shift of the cell-specific scramble code occurs.
As the Non-patent Document 5 indicates, when correlation is determined using the cell-specific pilot signals in the sub-frames #0 and #5 in which the synchronization channel is multiplexed, if the phase shift of cell-specific scramble codes has occurred, it is inevitable to perform blind detection since the phases of cell-specific pilot signals in sub-frame #5 are unknown, therefore the processing volume increases and detection probability deteriorates.